Isobutene is an important monomer in the manufacture of fuel additives, butyl rubber polymer, and antioxidants (Bianca et al., Appl. Microbiol. Biotechnol., 2012, 93, 1377-1387).
Manufacturers of goods using isobutene as feedstock depend on a number of petroleum-based sources, including (i) a C4 stream from a steam cracker separated from the butadiene, (ii) butene-butane fractions from a catalytic cracker and (iii) n-butane (from liquid petroleum gas) that is isomerized to isobutane and dehydrogenated to isobutene.
Given a reliance on petrochemical feedstocks and energy intensive processes, biotechnology offers an alternative approach via biocatalysis. Biocatalysis is the use of biological catalysts, such as enzymes or whole cells, to perform biochemical transformations of organic compounds.
Accordingly, against this background, it is clear that there is a need for sustainable methods for producing intermediates, in particular isobutene, wherein the methods are biocatalysis based. Both bioderived feedstocks and petrochemical feedstocks are viable starting materials for the biocatalysis processes.
The introduction of a double bond into a short branch chain aliphatic carbon substrate is a key consideration in synthesizing isobutene via a biocatalytic process. In this vain, a cytochrome P450 from Rhodotorula minuta var. texensis IFO 1102 forms isobutene via the decarboxylation of isovalerate. Also, it has been demonstrated that variants of oleate hydratase accept isobutanol and variants of mevalonate diphosphate decarboxylase accept 3-hydroxy-3-methylbutyrate as a substrate in the biosynthesis of isobutene. A number of enzymes have thus been identified as having catalytic activity in the synthesis of isobutene (Bianca et al., Appl. Microbiol. Biotechnol., 2012, 93, 1377-1387).
However, the identified biochemical pathways leading to the precursors accepted for isobutene synthesis are carbon inefficient, reflected by low maximum theoretical yields on carbon. For example, the only pathways identified for exploiting the catalytic activity of mevalonate diphosphate decarboxylase have maximum theoretical yields of ˜0.2 [(g isobutene)/(g glucose)] (Bianca et al., 2012, supra). The economical production of isobutene using mevalonate diphosphate decarboxylase as final enzymatic step is thus challenged by carbon yield limitations.